This is why your car thermometer is almost always wrong

ByGreg Porter

June 12, 2017

(iStock)

Few industries integrate technology into their products faster than do automobile companies. One of those features, now ubiquitous in most cars, is the dashboard thermometer display. But the temperature reading on your vehicle’s dashboard is often misleading and not representative of the actual temperature.

In its most basic definition, temperature is a measurement of how fast gas molecules are moving or its average kinetic energy. Heat excites molecules, causing them to bounce around much more frequently, resulting in a higher temperature value. The most common way to measure this reaction is with a mercury thermometer, where the liquid mercury will physically expand and rise to a particular value when heat is added.

Your car is equipped not with a thermometer but with a thermistor. Thermistors work in a similar manner to thermometers, but rather than using a liquid like mercury, thermistors measure the change in electrical current as a result of heat added or taken away. Thermistors are quite convenient, since they are small, cheap to make and for the most part, accurate. And really, the problem is not that your car uses a thermistor but rather where that thermistor is placed.

Most vehicles have their thermistors on the front of the car, located behind the grill. This location makes the instrument’s measurements sensitive to reradiated heat from the road surface. If you’ve ever walked barefoot across sand or concrete that’s been exposed to direct sunlight, then you’ve experienced reradiated heat directly.

Surfaces such as roadways are great absorbers of the sun’s incoming radiation. And consequently, they heat up very quickly, creating a localized hot spot right at the surface. As you can imagine, the temperature of a road surface in the middle of a hot summer day is not a true representation of the air temperature. But the heat generated is real, and it’s this “extra” heat that is picked up by a car’s thermistor, artificially inflating the reading you see on your dashboard.

So in effect, your car’s thermistor is measuring the temperature of a very localized environment and not what we would consider the air temperature. The average or ambient temperature that we associate with your daily highs and lows is measured in a sheltered, controlled environment, to limit the effects of heating from nearby surfaces as much as possible.

Car thermistors provide a better representation of air temperature at night than day because the sun’s heat isn’t reradiating off the road surface any longer. And they’ll also be a truer reflection of air temperature in cloudy daytime conditions (vs. sunny conditions) for the same reason.

The artificial heating effect of the road is further minimized when you’re traveling at high speeds vs. stop-and-go traffic.

During winter storms, the thermistors can be a helpful gauge as to how far below freezing it is and changing conditions as you coast through various microclimates. But they are not reliable enough to accurately discern temperatures within a degree of freezing, which can be the difference between wet roads and black ice.

Despite their shortcomings, car temperature readings are fun to monitor and can give you a sense of how fast temperatures are changing. Next time we get a burst of heavy rain from a thunderstorm, take a look at how much your car temperature drops.

Here’s a nice video explainer on car thermometers by Brad Panovich, chief meteorologist for the NBC affiliate in Charlotte:

Here's Why Your Car Thermometer is Wrong. I get more pictures every year of how hot your car "thinks" it is outside. Well here's the bad news it's warm just not that warm and here's why. #cltwx #ncwx #scwx

Greg PorterGreg holds a B.S. in meteorology from the University of Massachusetts at Lowell and an M.S. in meteorology from the University of Maryland. He currently works as an aviation meteorology consultant in Washington D.C.